1. Field of the Invention
The invention relates generally to a lubricant for lubricating journal bearings in a rock bit for drilling earth formations.
2. Background Art
Rock bits are employed for drilling wells in subterranean formations. Such bits have a body connected to a drill string and a single roller cone or a plurality (typically two or three) of roller cones mounted on the body for drilling rock formations. The roller cones are mounted on journals or pins integral with the bit body at its lower end. In use, the drill string and bit body are rotated in the bore hole, and each cone rotates on its respective journal as the cone contacts the bottom of the bore hole being drilled.
Drill bits are used in hard, often tough formations and, therefore, high pressures and temperatures are encountered. The total useful life of a drill bit is typically on the order of 20 to 200 hours for bits in sizes of about 6 to 28 inch diameter at depths of about 5,000 to 20,000 feet. Useful lifetimes of about 65 to 150 hours are typical. When a drill bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string to replace the bit which is a very expensive and time consuming process. Prolonging the lives of drill bits minimizes the lost time in “round tripping” the drill string for replacing bits.
Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. One reason for replacing the rock bits includes failure or wear of the journal bearings on which the roller cones are mounted. The journal bearings are subjected to very high drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures due to drilling, as well as elevated temperatures in the formation being drilled. The operating temperature of the grease in the drill bit can exceed 300° F. Considerable work has been conducted over the years to produce bearing structures and employ lubricants between the bearing surfaces that reduce friction, minimize wear and failure of such bearings.
A variety of grease compositions have been previously employed in attempts to reduce friction and thus reduce wear. U.S. Pat. No. 4,358,384 discloses one prior art grease composition that consists of a petroleum derived mineral oil lubricant basestock and a metal soap or metal complex soap including aluminum, barium, calcium, lithium, sodium or strontium metals. A lighter, lower-viscosity basestock is generally employed to obtain low temperature greases, and a heavier, higher-viscosity basestock is used to obtain high temperature greases.
Without being restricted to any method, in drilling applications, the mechanism of lubrication is by way of hydrodynamic lubrication. When at rest, the journal and the journal bearings of a drill bit squeeze out the lubricant and make direct contact. As the journal begins to rotate, the lubricant is drawn into the space between contacting surfaces to form a fluid wedge there between. As the journal rotation increases speed, this fluid wedge pushes the journal off the bearings and forms a lubricating film between the contacting surfaces. The film thickness is determined by both the rotation speed and load capacity of the lubricant. If a film is too thin, the asperities may make contact with a greater force, resulting in shearing action between the surfaces instead of a sliding action, which in turn generates heat and wears down the contacting surfaces.
In order to enhance the lubricating capacity of typical petroleum basestock greases, anti-wear agents have been typically added. The anti-wear agents, many of which function by a process of interactions with the metal surfaces, provide a chemical film which reduces or prevents metal-to-metal contact under high load conditions. U.S. Pat. Nos. 4,358,384, 3,062,741, 3,107,878, 3,281,355, and 3,384,582 disclose the use of molybdenum disulfide, and other solid additives such as copper, lead and graphite, which have been employed to attempt to enhance the lubrication properties of oils and greases.
Additives which are useful under extremely high load conditions are frequently called extreme pressure (EP) agents. These materials serve to enhance the ability of the lubricant base stock to form a friction-reducing film between the moving metal surfaces under conditions of extreme pressure and to increase the load carrying capacity of the lubricants. The function of the lubricant is to minimize wear and to prevent scuffing and welding between contacting surfaces. When metal asperities make contact with greater force and result in shearing rather than sliding, which in turn generates heat and wears down the contacting surfaces, EP additives in the lubricant are activated by the high temperature resulting from the extreme pressure to react with the exposed metal surfaces and form a protective coating thereon.
Additionally, while the basestock grease serves important functions with respect to friction and wear performance, it is generally inferior with respect to thermal conductivity. The thermal conductivity of oils, e.g., mineral oil, polyalphaolefins, ester synthetic oils, etc is typically in the range of 0.12 to 0.16 W/m*K, and water has a much higher thermal conductivity at 0.61 W/m*K. Many of the additives present in a lubricating composition may also act to improve the cooling capabilities as compared to a basestock alone. It is well known that metals in solid form have orders-in-magnitude larger thermal conductivities than those of fluids. For example, the thermal conductivity of copper at room temperature is about 3000 times greater than engine oil or pump oil. Therefore, typical lubricants containing such metallic particles generally exhibit significantly enhanced thermal conductivities relative to fluids alone.
Efforts to even further improve the thermal capacity of heat transfer fluids (coolants) have been attempted by varying the metallic additives, not just in type, but in size as well. The original studies of the thermal conductivity of suspensions were confined to those containing millimeter- or micron-sized particles. Maxwell's model shows that the effective thermal conductivity of suspensions containing spherical particles increases with the volume fraction of the solid particles. It is also known that the thermal conductivity of suspensions increases with the ratio of the surface area to volume of the particle. Using Hamilton and Crosser's model, it can be calculated that, for constant particle size, the thermal conductivity of a suspension containing large particles is more than doubled by decreasing the sphericity of the particles from a value of 1.0 to 0.3 (the sphericity is defined as the ratio of the surface area of a particle with a perfectly spherical shape to that of a non-spherical particle with the same volume). Because the surface area to volume ratio is 1000 times larger for particles with a 10 nm diameter than for particles with a 10 μm diameter, a much more dramatic improvement in effective thermal conductivity can be expected as a result of decreasing the particle size in a solution than can obtained by altering the particle shapes of large particles. While nanoparticles have been introduced in typical coolants, in the drilling industry the only nanoparticles used have been limited to carbon black, which shows a fairly low increase in thermal conductivity.
For additives to prove beneficial in a grease used in a drilling application, it is necessary to balance thermal performance, the load carrying capacity, and seal/glad wear. Generally, lubricants that reduce seal and gland wear typically lack sufficient film strength, that is, load carrying capacity, and lubricants with sufficient film strength tend show excessive seal and glad wear, to be used as a drill bit lubricant.
Accordingly, there exists a need for lubricant that exhibits improved thermal performance, a tight seal, and good load carrying capacity with reduced seal and gland wear.